Protein kinases are enzymes which covalently modify proteins and peptides by the attachment of a phosphate group to one or more sites on the protein or peptide. The measurement of protein kinase activity is important since studies have shown that these enzymes are key regulators of many cell functions.
The most widely used technique for measuring protein kinase activity is based on radioactive detection. In this method, a sample containing the kinase of interest is incubated with activators and a substrate in the presence of gamma .sup.32 P-ATP. After a suitable incubation period, the reaction is stopped and an aliquot of the reaction mixture is placed directly onto a filter which binds the substrate. The filter is then washed multiple times to remove excess radioactivity, and the amount of radiolabelled phosphate incorporated into the substrate is measured by scintillation counting.
This method is widely used and provides an accurate method for determining protein kinase activity in both crude and purified samples. However, because of the necessity of multiple washings, which are generally done by manually transferring the filter to a beaker and washing and rinsing with gentle agitation, the procedure is quite time consuming.
Other methods for detecting kinase activity are based on separations due to the charge differences between phosphorylated and non-phosphorylated proteins and peptides. In these respects, techniques based on gel electrophoresis and HPLC have, among others, been used. In combination with these techniques, spectrophotometric and fluorometric detection have been used. Reference is made to International Patent Application WO 93/10461 and U.S. Pat. Nos. 5,120,644 and 5,141,852 for descriptions of many methods heretofore used for detecting protein kinase activity. Also reference is directed to Analytical Biochemistry, 209, 348-353, 1993, "Protein Kinase Assay Using Tritiated Peptide Substrates and Ferric Adsorbent Paper for Phosphopeptide Binding."